A pilot study which employed a response-independent within-subject design with pigeons compared the aversive properties of two negative discriminative stimuli (CS negative) as measured by the amount of attack induced by each against a rear-projected image of a conspecific. Many responses (termed errors) occurred to the CS negative which had been associated with food prior to becoming a signal for nonreinforcement while very few errors occurred to the other CS negative. Significantly more attack was induced by the errorful CS negative than by the errorless CS negative. The proposed research will extend the pilot study by examining the role of errors and prior association with reinforcement in determining the aversiveness of discriminative stimuli which predict nonreinforcement. Experiment One will compare the attack-inducing properties of two CS negative which have had no prior association with food but differ markedly in the number of errors which occur to each. Experiment Two will compare the attack-inducing properties of two errorless CS negative when one of them has previously been a predictor of food.